1. Field of the Invention
The present invention relates to a brake disc in a disc brake used in a vehicle such as a motorcycle.
2. Description of the Related Art
As the conventional brake disc used in motorcycles, a brake disc 50 shown in FIG. 5 has been well known (See the Japanese Design Registration No. 1151976.). The brake disc 50 has a circular outer periphery and also has an inner peripheral portion adapted to be coupled with an outer peripheral portion of a disc hub 52 by a plurality of rivet-like pins 51 arranged in a circumferential direction. When the disc hub 52 is bolted to a hub (not shown) of the wheel, the brake disc 50 is supported by the wheel through the disc hub 52. A braking force is applied to the wheel when opposite braking surfaces of the brake disc 50 are sandwiched by a pair of frictional pads in a caliper fitted to a vehicle frame structure.
In the meantime, the brake disk 50 is required to reduce its outer diameter and plate thickness in view of the demand for reduction in weight. However, if the outer diameter and the plate thickness are reduced, the heat capacity and the amount of heat dissipation decrease, so that during braking the temperature of the brake disc 50 may increase to result in thermal deformation of the brake disc 50. Also, since the radial width of the braking surfaces of the disc 50 is fixed in the circumferential direction, brake squeal or noises tend to occur during braking as a result of resonance taking place between the brake disc 50 and the frictional pads.
In view of the above, there is known a brake disc having an outer peripheral surface formed with a circumferentially extending groove in order to secure the amount of heat dissipation, but the effectiveness of the groove is still insufficient. According to a series of experiments conducted by the inventor of the present invention, it was found that the effectiveness of the groove was something within the range of a measurement error. Also, since the radial width of the braking surfaces of the brake disc is fixed in the circumferential direction, the brake noises cannot be prevented.
In addition, as a brake disc for use in automotive vehicles that is designed to reduce the weight and increase the amount of heat dissipation, the brake disc disclosed in U.S. Pat. No. 6,386,340 is known in which the outer peripheral face and the inner peripheral face are formed in a sinusoidal waveform. However, the radial width of the braking surfaces of the brake disc remains fixed in the circumferential direction and, accordingly, the brake noises cannot be avoided.